The present invention relates to inclusion pigments having zirconium silicate as a casing component and an inorganic metal oxide having a fine particle size as an included color compound, wherein chromic oxide having a well-defined particle size is used as the inorganic metal oxide.
Furthermore, the present invention is directed to a process for forming the inclusion pigments which includes adding chromic oxide into zirconium silicate formed in a solid phase reaction.
The invention further relates to the use of inclusion pigments for the preparation of brilliant light green ceramic decorations.
Green chromic oxide (Cr.sub.2 O.sub.3) has long been used, due to its relatively good color and enamel stability, as a pigment for the coloring of ceramic masses and enamels, and also as a lower enamel color and a component of decoration colors in the preparation of ceramic decorations.
Brilliant light green shades of color are particularly desirable. As is the case with many other pigments, the shade of color of chromic oxide depends upon its average particle diameter (D.sub.50 value) and also upon the distribution of particle sizes. Desirable brilliant light green shades of color are obtained by using chromic oxide having an average particle size in the range of 0.1 to 3.0 .mu.m. In the CIE-Lab-system (German Industrial Standards Nos. 5033, 6164 and 6174), such chromic oxides show a color value in the range of L=30 to 35, a=-15 to -17 and b=13 to 16, measured with a 5% by weight coloration in a transparent enamel and by burning or firing at 1060.degree. C. Under identical test conditions, chromic oxide of a crude particle size gives color values of L=20 to 25, a=-12 to -14 and b=9 to 12. However, these chromic oxides having a crude particle size are not only of a darker color, but they also show a blue shade. This fact often represents a drawback.
Dark green chromic oxide having a crude particle size can be transformed into a light green pigment of fine particle size by grinding. Although the pigments thus obtained have the desired color, they have drawbacks from the standpoint of the technology of their application. More particularly, such pigments are less suitable for direct pressing in the preparation of ceramic decorations, due to their fine particle size. The fine details of the decorations are often smudged and/or the application of the decoration onto the ceramic carrier to be colored is imperfect. Non-included chromic oxide is susceptible to oxidation and formation of chromate in the lower enamel decoration, particularly at higher firing temperatures, and this results in drain traces during firing.